Piezoelectric pressure sensors have many applications such as underwater hydrophones, strain sensors and vibration sensors. Many types of piezoelectric sensors have been proposed regarding different piezoelectric materials, methods of construction and various other features.
For example, piezoelectric cables are disclosed in U.S. Pat. Nos. 4,278,000, 4,568,851 and 4,849,946. As the typical inner conductor, there are disclosed a single metal wire, a small twisted steel wire of about 0.3 mm in diameter and a conductive polymer layer. Since flexibility is very significant in the piezoelectric cable, piezoelectric polymers and piezoelectric composites are used as the piezoelectric material. As the typical piezoelectric polymer, polymers such as polyvinylidene fluoride(PVDF), polyvinyl difluoride/tri-fluoride ethylene copolymer (VDF/TrFE) and the like are disclosed. As the typical piezoelectric composite, composites such as silicone rubber, chloroprene rubber, urethane rubber and the like containing piezoelectric ceramic powders such as lead titanate (PbTiO3), solid solution of lead titanate-lead zyrconate (PbTiO3--PbZrO3) and the like are disclosed. As the outer conductor, there are disclosed a vacuum deposited metal film, a coated film of a conductive paste, a metal foil, a conductive rubber or polymer layer and others. As the protective jacket, polyethylene or polyvinyl chloride are known to be used.
The piezoelectric composites described above are often required to be cured to increase the physical strength and thermal stability. For example, a composite comprising chloroprene rubber and lead titanate is cured at 170.degree. C. under a pressure of 150 kg/cm.sup.2 after they are mixed, as reported in "Huntai to Kogyo" Vol. 22, No. 1, p. 53-54 , 1990. Other composites are also cured, as reported in 1984 Ultrasonic Symposium, p. 504 and "Ferroelectronics", Vol. 77, p. 39 , 1988. Since curing requires a particular apparatus and process, the composite is preferably produced without curing.
The conventional piezoelectric polymers and composites described above are thermally stable at a temperature less than 70-100.degree. C. However, when the coaxial cables are applied in temperature applications above 100.degree. C., for example in a electrical heating blanket, a temperature near a heating wire increases to about 120.degree. C. at maximum. In this case, high thermal stability is required at 120.degree. C.
Vacuum deposited or coated films are one of the most suitable outer conductors because of their low mechanical impedance. However, when producing these films, an expensive apparatus and complicated processes are required. For example, when a vacuum deposited metal film is produced, there are required a vacuum depositing apparatus equipped with an evacuating system, an evaporating system and other necessary apparatuses. Moreover, many processes are required such as valve operations and deposition operations to deposit the metal film around the circumferential surface of the piezoelectric layer under suitable conditions such as cleaning the surface of the piezoelectric layer, deposition rate control, deposition temperature control, film thickness control and others. When a coated film is used, there are also required an expensive coating apparatus and several complicated processes. When a thin metal foil is used as the outer conductor, an expensive apparatus and complicated processes are not required. However, the metal foil is problematic because it is often difficult to firmly wind the metal foil around the circumferential surface of the piezoelectric layer because of the weak physical strength of the metal foil. For example, a thinner aluminum foil less than 15 .mu.m in thickness is easily broken by hand. The conductive rubber or polymer layers are advantageous because these layers can be produced by the same extruding process as the piezoelectric and protective layers. However, when the rubber or polymer layers are used as the outer conductor, an expensive extruding apparatus and complicated processes are required.
On the other hand, many planar pressure sensors have also been developed. As the piezoelectric material, piezoelectric ceramics such as lead titanate (PbTiO3), solid solution of lead titanate-lead zyrconate (PbTiO3--PbZrO3) and the like are also used together with piezoelectric polymers and piezoelectric composites described above, as shown in "Piezoelectric Ceramic" (Bernard Jaffe, William R. Cook Jr. and Hans Jaffe, Academic Press, 1971, p. 262). Metal films such as gold, palladium and platinum have been fired or evaporated on the piezoelectric ceramics as the conductor. However, the piezoelectric ceramics cannot be applied to a curved surface because of their brittle property.